First you need to find the dcrwallet application directory which varies by platform.
This is the same directory that is home for your configuration files.

In that directory, you need to delete the file mainnet/wallet.db.
That will completely remove your wallet from that computer. To access
it again you will need to restore from the original seed.

It is important to note that it is possible (but not certain) that a
deleted file may be recovered using file recovery tools. If you are
deleting this for security reasons you probably need to use a secure
deletion tool such as
GNU shred.